RF power amplifiers used for wireless communication transmitters, with spectrally efficient modulation formats, require high linearity to preserve modulation accuracy and to limit spectral regrowth. Typically, a linear amplifier, Class-A type, Class-AB type or Class-B is employed to faithfully reproduce inputs signals and to limit the amplifier output within a strict emissions mask. Linear amplifiers are capable of electrical (DC power in to RF power out or DC-RF) efficiencies greater than 50% when operated at saturation. However, they are generally not operated at high efficiency due to the need to provide high linearity. For constant envelope waveforms, linear amplifiers are often operated below saturation to provide for operation in their linear regime. Time varying envelopes present an additional challenge. The general solution is to amplify the peaks of the waveform near saturation, resulting in the average power of the waveform being amplified at a level well backed-off from saturation. The back-off level, also referred to as output power back-off (OPBO), determines the electrical efficiency of a linear amplifier.
For example, the efficiency of a Class-A type amplifier decreases with output power relative to its peak value (EFF=POUT/PPEAK). The efficiency of Class-B type amplifiers also decreases with output power relative to its peak value (EFF=POUT/PPEAK)1/2). Class-AB type amplifiers have output power variations intermediate between these values. Thus, there is customarily an inherent tradeoff between linearity and efficiency in amplifier designs.
Modern transmitters for applications such as cellular, personal, and satellite communications employ digital modulation techniques such as quadrature phase-shift keying (QPSK) in combination with code division multiple access (CDMA) communication. Shaping of the data pulses mitigates out-of-band emissions from occurring into adjacent channels but produces time-varying envelopes. In addition to amplifying individual waveforms with time varying envelopes, many transmitters (especially in base stations) are being configured to amplify multiple carriers. Multi-carrier signals have high a wide distribution of power levels resulting in a large peak-to-average ratio (PAR). Therefore, the operation of the linear amplifiers in these types of signals is very inefficient, since the amplifiers need to have their supply voltage sized to handle the large peak voltages even though the signals are much smaller a substantial portion of the time. Additionally, the size and cost of the power amplifier is generally proportional to the required peak output power of the amplifier.
Wideband Code Division Multiple Access (WCDMA), Orthogonal Frequency Division Multiplexing (OFDM), and multi-carrier versions of Global Standard for Mobile Communication (GSM) and Code Division Multiple Access 2000 (CDMA 2000) are wireless standards and application growing in use. Each requires amplification of a waveform with high PAR levels, above 10 dB in some cases. The sparse amount of spectrum allocated to terrestrial wireless communication requires that transmissions minimize out-of-band (OOB) emissions to minimize the interference environment. A linear amplifier used to amplify a waveform with a PAR of 10 dB or more provides only 5–10% DC-RF efficiency. The peak output power for the amplifier is sized by the peak waveform. The cost of the amplifier scales with its, peak power. Several other circuit costs including heat sinks and DC-DC power supplies scale inversely to peak power and dissipated heat (which results from the electrical inefficiency). Related base station costs of AC-DC power supplies, back-up batteries, cooling, and circuit breakers also scale inversely with efficiency as does the electrical operating costs. Clearly, improving DC-RF efficiency is a major cost saver both for manufacture and operation.
One efficiency enhancement technique for linear amplifiers is known as envelope tracking or envelope following. In an envelope following or envelope tracking, the supply voltage to a linear power amplifier is reduced or increased in response to the amplitude of the amplitude modulated envelope. The supply voltage applied to the power amplifier follows the amplitude modulated envelope of the input signal provided to the power amplifier. The supply voltage is maintained at levels that assure amplifier operation out of saturation. For example, when the envelope amplitude is at peak, the supply voltage is at a signal level above the peak voltage, and when the envelope amplitude is at its minimum, the supply voltage decreases below the peak voltages, thus providing a more efficient amplifier than constant supply linear amplifiers. However, the envelope following typically requires a significant output power back-off (OPBO) level or headroom compared to peak signal levels to achieve high linearity (e.g., ensuring the peak of the input signal is not distorted by amplifier gain compression). This requires much larger, more costly amplifiers to provide the necessary back-off and thus lower efficiency. Additionally, if the signal varies over a large dynamic range it may cause the envelope tracking amplifier to operate with very low drain voltages resulting in additional signal distortion and eventually causes the amplifier system to shutoff.
A second efficiency enhancement is known as the Kahn or Envelope Elimination and Restoration (EER) technique. The EER technique detects the envelope of the incoming signal to produce a baseband amplitude modulated (AM) signal. The EER technique limits the input signal to produce a constant envelope phase modulated (PM) component. The PM signal is provided to the input of the power amplifier along a PM path (typically applied to the Gate or Base of a transistor) and the amplitude modulated component is employed to modulate the supply of the power amplifier (typically at the Drain or Collector) along an AM path. Amplitude modulation of the final RF power amplifier restores the envelope to the phase-modulated carrier, creating an amplified version of the original modulated input signal. Since the signal input to the power amplifier has a generally constant envelope amplitude, a more efficient class of power amplifier (e.g., Class-C type amplifiers) can be employed to amplify the input signal. Additionally, since the supply signal is amplitude modulated, the amplifier is operating at compression enhancing the overall efficiency of the amplifier. Amplifiers using EER are known as Polar Amplifiers.